Cause

If you are trying to install package manually via PPA web interface, you will be able to select only 32bit package, which is not compatible with management node.

Resolution

It is not necessary to install webmail package on PPA management node manually, because it is installed by default.
You can read more about services on management node in Parallels Plesk Automation Deployment Guide, on page 5 "Hosting Services Provided by the Management Node".